1. Technical Field
The present invention relates to a disk drive, an actuator, and a stator magnet configuring a voice coil motor (hereinafter, a VCM) of the actuator, and in particular, to a configuration for improving a breathing phenomenon of a coil occurring during the operation of the VCM.
2. Description of the Related Art
FIG. 10 is a schematic of an actuator 100 used in a conventional hard disk drive. An actuator arm 101 is configured by a suspension 102 and a coil support part 103 in one piece, is rotatably supported by a rotary shaft 104 setting on a base (not shown), and is driven by a VCM, described later, in the direction shown by an arrow J or K.
A slider 109 is supported in an edge of the suspension 102, and respective heads for reading and writing that are not shown are provided on this slider 109. When the actuator arm 101 is positioned on a recording surface of a hard disk (not shown) rotating, the actuator 100 is configured so that the heads face the recording surface with keeping a predetermined gap between the recording surface and themselves by the slider 109 flying over the recording surface of the disk.
In the actuator arm 101, the slider 109 is supported in the edge of the suspension 102 as described above. Nevertheless, a pair of coil supports 103a and 103b for sandwiching a flat coil 105 configuring the VCM is formed in the coil support part 103 positioned in the opposite side of the slider 109 against the rotation shaft 104. A lower stator magnet retention plate 106 fixed on the base retains a stator magnet 107 below the flat coil 105. The stator magnet 107 has a north pole 107a and a south pole 107b, and these are formed with making a boundary 107c a borderline. The VCM is configured by these flat coil 105 and stator magnet 107, and the actuator 100 is configured by this VCM and the actuator arm 101.
In the configuration described above, the flat coil 105 obtains a force in a rotational direction shown by an arrow H in each of the side edges 105a and 105b. This is because the flat coil 105 is located so that an electromagnetic action may occur between the flat coil 105 and stator magnet 107. Therefore, the actuator arm 101 obtains a rotary force in a clockwise direction if current in the direction shown by an arrow m passes through the flat coil 105. On the contrary, if the current passes through the flat coil 105 in the direction shown by an arrow n, the actuator arm 101 obtains a rotary force in the counterclockwise direction. This is because the flat coil 105 obtains a force in the rotary direction shown by an arrow I in each of the side edges 105a and 105b. 
On the other hand, an outer edge 105c of the flat coil 105 is not supported by the coil support part 103 because of lightening and miniaturizing the coil support part 103, and further making a torque small. Nevertheless, the outer edge 105c receives a force in a radial direction shown by an arrow F or G according to the direction of the current passing and its rotary position.
FIGS. 11 and 12 are operational diagrams for explaining a force that the outer edge 105c of the flat coil 105 receives, but the suspension 102 of the actuator arm 101 (FIG. 10) is omitted. FIG. 11 shows such a state that the actuator arm 101 is present at a position (hereinafter, this is called an OD position) where the actuator arm 101 rotates at most in the direction, shown by an arrow H, within its rotatable range. At this position, the outer edge 105c of the flat coil 105 is present above the north pole 107a of the stator magnet 107. Therefore, if current in the direction shown by an arrow m passes through the flat coil 105, the outer edge 105c receives a force in the direction shown by an arrow F that heads from the shaft center of the rotary shaft 104 to the outside. On the contrary, if current in the direction shown by an arrow n, the outer edge 105c receives a force in the direction shown by an arrow G that heads toward the shaft center of the rotary shaft 104.
FIG. 12 shows such a state that the actuator arm 101 is present at a position (hereinafter, this is called an ID position) where the actuator arm 101 rotates at most in the direction, shown by an arrow I, within its rotatable range. At this position, the outer edge 105c of the flat coil 105 is present above the south pole 107b of the stator magnet 107. Therefore, if current in the direction shown by an arrow m passes through the flat coil 105, the outer edge 105c receives a force in the direction shown by an arrow G. On the contrary, if current in the direction shown by an arrow n, the outer edge 105c receives a force in the direction shown by an arrow F.
FIGS. 13 and 14 are drawings of analyzing the deformation of the flat coil 105 and coil supports 103a and 103b, sandwiching the flat coil 105, when the flat coil 105 resonates at a predetermined frequency by alternately receiving forces in the directions shown by No arrows F and G, by numerical simulation using a finite-element method (FEM). As shown in FIG. 13, when the outer edge 105c of the flat coil 105 protrudes in the direction shown by an arrow F and hence the flat coil 105 is extended, an angle between the coil supports 103a and 103b sandwiching this decreases. On the other hand, as shown in FIG. 14, when the outer edge 105c of the flat coil 105 dents in the direction shown by an arrow G and hence the flat coil 105 is shrunk, an angle between the coil supports 103a and 103b sandwiching this increases.
Such a vibration mode wherein a coil is extended and shrunk is called a coil-breathing mode. A piezoelectric element 108 (FIG. 10) detects an amount of extension or shrinkage of the coil support 103b where the piezoelectric element 108 is fixed. In addition, as FIG. 13, the piezoelectric element 108 detects extension when the flat coil 105 is extended and hence the angle between the coil supports 103a and 103b decreases. Furthermore, the piezoelectric element 108 outputs, for example, plus voltage at a level according to the extension amount. On the contrary, as shown in FIG. 14, the piezoelectric element 108 detects shrinkage when the flat coil 105 is shrunk and hence the angle between the coil supports 103a and 103b increases. Furthermore, the piezoelectric element 108 outputs, for example, minus voltage at a level according to the shrinkage amount. In addition, a fixed position of the piezoelectric element 108 (FIG. 10) is determined so that it is possible to detect warpage occurring when the actuator arm 101 receives acceleration in a rotary direction.
FIGS. 15a and 15b show frequency characteristics of a transfer function from the drive current of the flat coil 105 to the output voltage of the piezoelectric element 108 in the actuator 100 (FIG. 10) configured as described above. In the frequency characteristic charts, the horizontal axis shows frequencies from 2 kHz to 16 kHz that are linearly graduated. In addition, the vertical axis in FIG. 15(a) shows gains expressed in decibels, and the vertical axis in FIG. 15(b) shows phases. Furthermore, dotted lines show frequency characteristics of a transfer function A2od(s) at the time when the actuator arm 101 is near the OD position shown in FIG. 11. Moreover, continuous lines show frequency characteristics of a transfer function A2id(s) at the time when the actuator arm 101 is near the ID position shown in FIG. 12.
As being apparent from FIG. 15, although the actuator 100 resonates at nearly 4 kHz, this is butterfly resonance caused by the warpage of the actuator arm 101. In addition, although the phase largely changes near this frequency, two phases at different rotary positions of the actuator arm, that is, the OD position and ID position become the same.
On the other hand, resonance at nearly 10 kHz is coil-breathing resonance caused by the coil breathing described above. In this resonance, the phases at different rotary positions, that is, the OD position and ID position become opposite. This is because directions of the forces that the outer edge 105c receives become opposite against the current passing through the outer edge 105c since polarities of the stator magnets that the outer edge 105c of the flat coil 105 faces at the OD position and ID position are different.
Technology of actively damping the above-described butterfly resonance is disclosed in Japanese Patent Application No. 11-80723 filed by the present applicant. According to this, in a hard disk drive, by not only performing tracking control to drive a VCM of an actuator so as to make heads positioned above a desired track, but also driving the VCM of the actuator in the direction where warpage is removed through detecting a warpage component of the actuator with the above-described piezoelectric element, the stability of the tracking control is improved.
Owing to this, a control signal for the tracking control and a control signal for damping the butterfly resonance are superimposed, and the current passing through a flat coil configuring the VCM on the basis of this signal superimposed is controlled. Nevertheless, if such damping control technology is applied to an actuator having frequency characteristics shown in FIGS. 15a and 15b, various problems arise. Thus, in a frequency band of 10 kHz or higher, coil breathing has large effect, and hence phases near rotary positions of the actuator arm 101, that is, the OD position and ID position are largely different. In particular, in nearly 10 kHz, and 14 kHz and higher, respective phases become opposite, and hence it is impossible to make stable control near both rotary positions compatible.
In addition, there is a method for removing a high frequency range, where the coil breathing has effect, by a filter in a control loop. Nevertheless, in the actuator 100 (FIG. 10) that has a wide rotation angle and is used in a hard disk drive, it is not possible to narrow the width of the outer edge 105c of the flat coil 105. Therefore, the resonance frequency of the coil breathing becomes low, and hence is present near a butterfly resonance frequency. Therefore, it is difficult to remove only this part by a filter.
Furthermore, so as to enlarge the torque of an actuator, it is common to extend magnetic poles of a stator magnet to a moving area of the outer edge 105c of the flat coil 105 as shown in FIG. 10. Nevertheless, even if the magnetic poles are configured, for example, for damping the coil-breathing phenomenon so that the magnetic poles may be not extended to this moving area, the flat coil 105 is affected by leakage magnetic flux from adjacent north and south magnetic poles. Therefore, it is difficult to damp the coil-breathing phenomenon at a level where the phenomenon has no effect on the control.
Thus, it is an object of the present invention to provide an actuator that can provide stabilized control of an actuator arm regardless of a rotary position of the actuator arm if the butterfly resonance and further coil breathing resonance are actively damped.
A stator magnet according to a first embodiment of the present invention forms a voice coil motor with a closed coil. The closed coil is supported by an actuator arm in a rotatable manner and at a predetermined rotation angle. The closed coil has first and second side edges that extend along different lines in radial directions whose center is a center of rotation of the actuator arm. It also has an outer edge that connects edges of outer sides of the first and second side edges with viewed from the radial direction, and extends along an arc whose center is the center of rotation. A first magnetic pole region is located within a moving area of the first side edge to act on the first side edge. A second magnetic pole region is located within a moving area of the second side edge to act on the second side edge. The polarity of the second magnetic pole region is opposite to the polarity of the first magnetic pole region. A third magnetic pole region is located within a moving area of the outer edge to act on the outer edge. The polarity of the third magnetic pole region is the same as the polarity of the first magnetic pole region.